# std::countl_one

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< cpp‎ | numeric

Defined in header `<bit>` |  |
template< class T > constexpr int countl_one( T x ) noexcept; |  | (since C++20)

Returns the number of consecutive 1 ("one") bits in the value of x, starting from the most significant bit ("left").

This overload participates in overload resolution only if `T` is an unsigned integer type (that is, unsigned char, unsigned short, unsigned int, unsigned long, unsigned long long, or an extended unsigned integer type).

### Parameters

x | - | value of unsigned integer type

### Return value

The number of consecutive 1 bits in the value of x, starting from the most significant bit.

### Notes

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_bitops` | `201907L` | (C++20) | Bit operations

### Example

Run this code

    #include <bit>
    #include <bitset>
    #include <cstdint>
    #include <iostream>

    int main()
    {
        for (const std::uint8_t i : {0, 0b11111111, 0b01111111, 0b11100011})
            std::cout << "countl_one( " << std::bitset<8>(i) << " ) = "
                      << std::countl_one(i) << '\n';
    }

Output:

    countl_one( 00000000 ) = 0
    countl_one( 11111111 ) = 8
    countl_one( 01111111 ) = 0
    countl_one( 11100011 ) = 3

### See also

countl_zero (C++20) | counts the number of consecutive ​0​ bits, starting from the most significant bit (function template)
countr_zero (C++20) | counts the number of consecutive ​0​ bits, starting from the least significant bit (function template)
countr_one (C++20) | counts the number of consecutive 1 bits, starting from the least significant bit (function template)
popcount (C++20) | counts the number of 1 bits in an unsigned integer (function template)
has_single_bit (C++20) | checks if a number is an integral power of 2 (function template)
count | returns the number of bits set to true (public member function of `std::bitset<N>`)
allanynone | checks if all, any or none of the bits are set to true (public member function of `std::bitset<N>`)

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